Introduction: is probably the most ancient administered

Introduction:

Gold is a chemical element with symbol Au (from Latin: aurum) and atomic number 79. In its purest form, it is a bright,
slightly reddish yellow, dense, soft, malleable and ductile metal.
The pure metal melts at 1063°C and boils at 2966°C.It has an atomic weight of
196.967 with a density of 19.32 g cm?3
at 20°C. Its beauty and rarity has led to the use in jewelry and in coinage. Chemically, gold is a transition metal and a group 11 element. Common oxidation state of
gold is +III and +I, although it can show oxidation states from –I to + V. It
is one of the least reactive chemical elements, and is solid under standard conditions. The metal therefore occurs
often in free elemental (native) form. Gold resists attacks by individual acids, but it can be dissolved by aqua regia (1:3 mixture of nitric acid and
hydrochloric acid). The acid mixture causes the formation of a soluble gold tetrachloride anion. Gold metal also dissolves
in alkaline solutions of cyanide, which are used in mining and electroplating. It is insoluble in nitric acid, which dissolves silver and base metals.
Its insolubility in nitric acid is used to refine it from silver or other base
metals.

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Gold because of its malleability and
ductility is used in various purposes such as infra-red shields, heat resistant
suits.

Gold as we know is probably the most ancient
administered medicine.  In the 19th century gold was used as a “nervine,” therapy for
nervous disorders. Depression, epilepsy, migraine, and glandular problems such as amenorrhea and impotence were treated with the help of gold. 1

Some gold salts do have anti-inflammatory properties and at present two are still
used as pharmaceuticals in the treatment of arthritis and other similar
conditions in the US (sodium aurothiomalate and auranofin). These drugs have been explored as a
means to help to reduce the pain and swelling of rheumatoid arthritis, and also (historically) against tuberculosis and some parasites 2

 Gold Nanoparticles because of their unique
optical, electronical and molecular recognition properties are used widely in
biological fields for various purposes.  Nanoparticles can be produced through various methods. Chemical process
are the most popular methods for nanoparticle production but some of the
chemical processes use toxic chemicals and hence can not be used for biological
purposes. Synthesis of Au nanoparticles using microbial, plant, plant extracts
and enzymes are the suggested eco-friendly ways. There are many microbes which
are known to produce nanostructured mineral crystals and metallic nanoparticles
with properties similar to chemically synthesised materials, while exercising
strict control over size, shape and composition of the particles.

 A fungus
Verticillium sp. when exposed to aqueous AuCl4? ions results in reduction of the metal
ions and formation of gold nanoparticles of around 20 nm diameter.3 The gold nanoparticles formed are reported to be on both the surface and
within the fungal cells (on the cytoplasmic membrane) with negligible reduction
of the metal ions in solution.4

Many
bacteria such as Thermomonospora sp, 5  Rhodococcus sp. , Rhodopseudomonas
capsulate ,6
Pseudomonas aeruginosa, 7Delftia acidovorans 8 produce gold nanoparticles by the process of
reduction of gold salt. Rhodococcus sp. is reported to produce gold
nanoparticle intracellularly 9 while others produce extracellularly.

Plant
extract have also been used for the preparation of gold nanoparticle. For
example, extract of Cymbopogon flexuosus form
nanoparticles extracellularly 10 while live alfalfa plant produce intracellularly 11.  One of the advantage of using plant for
nanoparticle production instead of using bacteria and fungi is the lack of
pathogenicity of plant nanoparticles. These organisms probably form
nanoparticles as a surviving mechanism adapted by the organism to cope with the
high levels of metal in the environment. The mechanisms may involve alteration
of the chemical nature of the organisms so that the metal no longer causes
toxicity. In this process the metal can be reduced and nanoparticle can be
produced. Thus, it can be said that the production of nanoparticle is
by-product of resistance mechanism evolved by the organism against the specific
metal, and therefore this process is exploited for the green synthesis of
nanoparticle.

To
date, there are numerous techniques for synthesizing nanoparticles. However,
these techniques fall into two broad approaches and can be defined as either a
top down approach or a bottom up approach 12. The top down approach
starts with a material of interest, which then undergoes size reduction via
physical and chemical processes to produce nanoparticles. Importantly,
nanoparticles are highly dependent on their size, shape, and surface structure
and processing tends to introduce surface imperfections. These surface
imperfections can significantly impact on the overall nanoparticle surface
physicochemical properties 13. In the bottom up approach, nanoparticles
are built from atoms, molecules and smaller
particles/monomers 1415 16. In either approach, the
resulting nanoparticles are characterized using various
techniques to determine properties such as
particle size, size distribution, shape, and surface area. This is of
particular importance if the properties of nanoparticles
need to be homogeneous for a particular application. 17

During
nanoparticle synthesis the gold salt is reduced by any reducing agent whether
chemical or biological which results into change in colour. This is the first
qualitative indication that the nanoparticle is formed. For further study of
nanoparticles various spectroscopy and microscopy techniques are being used
such as UV-Visible spectroscopy, dynamic light scattering (DLS), atomic force
microscopy (AFM), transmission electron microscopy (TEM),
scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS),
powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy
(FT-IR), and Raman spectroscopy. Microscopy based techniques
such as AFM, SEM and TEM obtain data from images taken of the nanoparticles. In
particular, both SEM and TEM have been extensively used to determine size and
morphological features of nanoparticles. While spectroscopy based techniques
such as UV-vis, DLS, XRD, EDS, FT-IR, determine data related to composition,
structure, crystal phase and properties of nanoparticles. In case UV-Visible spectroscopy
wavelengths between 300 and 800 nm are generally used for characterizing
metallic nanoparticles ranging in size from 2 nm up to around 100 nm 18. For
example, gold (Au) nanoparticles are generally detected by the presence of
peaks between 500 and 550 nm.19

 DLS spectroscopy is used to determine size
distribution and quantify the surface charge of nanoparticles suspended in a
liquid.
19 20

Researchers
have shown that different pH can also affect the formation of nanoparticles.
Importance of pH in the biosynthesis of colloidal gold using alfalfa biomass
have been shown and it is being concluded that the size of nanoparticles
changes with the change in pH. 21 Large nanoparticles are produced at lower
pH(2-4) 22.

 In case of Avena
sativa, it has been shown that formation of gold nanoparticle is highly
dependent on the pH value. At pH 2, large-sized nanoparticles (25?85 nm) are
reported, although in low quantity. At pH 3 and 4, smaller-sized nanoparticles
in a large quantity are reported. They speculated that at low pH (pH 2), the
gold nanoparticles prefer to aggregate to form larger nanoparticles rather than
to nucleate and form new nanoparticles. In contrast, at pH 3 and 4, more
functional groups (carbonyl and hydroxyl) are available for gold binding; thus,
a higher number of new Au (III) complexes would bind to the biomass at the same
time that will nucleate separately and form nanoparticles of relatively small
size. 23

Temperature
can also affect the synthesis of nanoparticle. It is reported that at higher
temperatures rate of formation of nanoparticle increases. 24 They
have also reported synthesis of nanorod and platelet-shaped gold nanoparticles
at higher temperatures and formation of spherical-shaped nanoparticles at lower
temperatures. Therefore, it can be concluded that temperature is also one of
the crucial factors which determine the size and shape of nanoparticles.

The
exact mechanism how the nanoparticles are formed is not understood still. It is
required to understand so that controlled and definite size and shaped
nanoparticle can be synthesized. 

 

 

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